Quote Originally Posted by Legendary Caladbolg
Charge up all your aeon's overdrives before you fight him.
Quickest and easiest method, I'd say. In my first game I had to resort to this, because I have a habit of being horrendously over-levelled. In my second game, I didn't have that much trouble. Aeon's are useful for taking power hits and dealing a lot of damage. For example Seymour's Lance of Astrophy and Life 2 combo won't work on an Aeon, because they're immune to undead.

I'd also suggest that if it's his powerful attack (I forget what it's called) that's giving you trouble, as soon as he hints at using it (I'm pretty sure some indication is given), summon an Aeon to take the hit for you.

I'm sorry I can't be that much help, I haven't played the game in quite some time.